1. Field of the Invention
The present invention relates generally to fuel assemblies for use in nuclear reactors and, more particularly, is concerned with a gage and inspection method for determining the orientation of bulges connecting grid sleeves with guide thimbles in fuel assemblies.
2. Description of the Prior Art
A typical pressurized water nuclear reactor contains a large number of fuel assemblies in its core. Each fuel assembly in the reactor core has the same construction. Basically, a fuel assembly is composed of a bottom nozzle, a top nozzle, an instrumentation tube and pluralities of guide thimbles, fuel rods and grids. The guide thimbles extend vertically between the bottom and top nozzles and rigidly interconnect them. The grids are attached in axially spaced positions along the guide thimbles such that a multiplicity of cells defined by interleaved straps of the respective grids are disposed in vertical alignment. The fuel rods are supported in an organized and transversely spaced array in the vertically aligned cells of the transverse grids by springs and dimples on the straps which extend into the cells.
The grids have short sleeves which extend above and below the interleaved straps for receiving therethrough the guide thimbles and providing means for attaching the grids thereto. Typically, concentric portions of the guide thimbles and grid sleeves are deformed or bulged together, such as by plastic expansion, to form mechanical expansion joints therebetween above and below the respective grids. In assembling the fuel assembly, the grids are attached to the guide thimbles beginning adjacent the lower ends thereof first and proceeding to adjacent the upper ends thereof. When subsequently the fuel rods are loaded through the grids, each guide thimble is surrounded by four fuel rods. The guide thimbles are typically larger in diameter than the fuel rods and so the space between a guide thimble and the adjacent fuel rods is less than between adjacent fuel rods. This reduction in the spacing between each guide thimble and its adjacent fuel rods reduces the amount of coolant flow along the sides of these fuel rods facing the guide thimble and thereby the amount of heat transfer from the fuel rods to the flowing coolant. This situation is termed hot channel factor which creates a departure for natural boiling (DNB) condition.
Therefore, in creating external bulges in the coaxially fitted grid sleeves and guide thimble to connect the same together, it is important to not aggravate the DNB condition by reducing the gap between the fuel rods and the guide thimble any further. Thus, it is most desirable to form each set of four bulges at 45-, 135-, 225- and 315-degree positions (hereinafter referred to as 45-degree reference points) about the guide thimble relative to placement of the fuel rods at 0-, 90-, 180- and 270-degree positions (hereinafter referred to as X-Y axes reference points) about the guide thimble. Accepted standards allow a maximum deviation from the 45-degree reference points of .+-.11 degrees at one bulge location and .+-.6 degrees at the rest of the bulge location.
The conventional process used to produce the expansion joints at each grid location forms each set of bulges at the same orientation from one grid to the next. Therefore, quality control personnel only need to inspect the expansion joints at the uppermost grid to determine whether the sets of bulges are located within the allowed range of deviation from the desired 45-degree reference points. Heretofore, the inspection method of verifying expansion joint orientation has been carried out through application of visual standards.
One major shortcoming of this inspection method is that it is too dependent on the subjective judgment of the personnel carrying it out. Consequently, a need exists for an approach to bulge orientation determination which will yield more objective or positive results, being independent of the skill of the particular inspector.